In vivo preclinical assays are required to screen potential agents that target the tumor vasculature. Here, a hollow fibre-based assay for the quantification of neovasculature in the presence or absence of an agent that potentially targets tumor neovasculature is described. The neovasculature is developed as a consequence of the presence of tumor cells encapsulated in hollow fibres, which are transplanted subcutaneously in the dorsal flanks of mice.